1. Field of the Invention
This invention relates generally to microwave and millimeter wave signal filters and more particularly to such filters which utilize slot transmission lines as the propagating medium.
2. Description of the Prior Art
Slot line filters and the characteristics of slot transmission lines are well known and described, for example, in: IEEE Transactions On Microwave Theory And Techniques, "Slot Line Characteristics", Elio A. Mariani, et al., Vol. MTT-17, No. 12, December, 1969, pp. 1091-1096; and IEEE Transactions On Microwave Theory And Techniques, "Slot-Line Filters And Couplers", Elio A. Mariani, et al., Vol. MTT-18, No. 12, December, 1970, pp. 1089-1095. As disclosed in those two articles, a slot transmission line comprises an elongated, relatively narrow slot formed in a metal coating applied to a dielectric substrate. This structure provides a transmission line for application to microwave integrated circuits. The slot line offers a unique combination of a planar type geometry and a TE dominant mode similar to the dominant mode of a rectangular waveguide. Furthermore, the slot line can be combined with microstrip circuitry utilizing a common dielectric substrate whereby two transmission lines are coupled through the dielectric medium.
Where a filter configuration is desired, the article entitled, "Slot Line Filter Couplers", proposes configuring a bandpass filter, for example, by a pair of quarter wave coupled resonant slots or a pair of end coupled resonant slots fabricated on a dielectric substrate. Coupling to and from the resonant slots can be achieved by either of two means: using a grounded center conductor of a semi-rigid coaxial cable traversing the slot and forming a magnetic coupling loop thereby or using a length of microstrip transmission line traversing the slot on the opposite side of the substrate.
Where a tunable filter is desired, the current state of the art proposes the use of the well known magnetically-tunable, yttrium-iron-garnat (YIG) filter; however, such structures result in relatively complex, costly coaxial circuit configurations which are non-planar.
Accordingly, it is an object of the present invention to provide an improvement in slot transmission line filters.
It is a further object of the invention to provide a tunable slot transmission line filter.
It is yet a further object of the invention to provide a tunable slot transmission line filter which provides relatively narrow-band frequency selectivity, i.e. pass-band, over a relatively broad operating band in the microwave and millimeter wave frequency range.